Vulkan is needed because the performance trade offs made by OpenGL / direct3D aren’t appropriate for some graphics engines. Vulkan allows knowledgeable & well funded developer teams (like Unreal) to make their own abstraction layer on top of the graphics card’s capabilities. And in doing so, get more performance out of their engines.

Having only OpenGL/D3D was like if the only way you could do network programming was via python. Python is easy enough to use but it’s not well optimized in every use case. Vulkan is like the C of graphics programming. It’s useful when you want to get in there at a lower level, because the higher level abstractions leave too much performance on the table.